It is known to form conductor patterns on one or both sides of a number of individual layers of dielectric material, and then to stack and unite such patterned layers to produce a multilayer circuit board. A typical dielectric material used in such multilayer circuit boards is an epoxy-glass material, while the conductor pattern is generally formed from copper, e.g., by selective etching of a thin layer of copper initially covering a layer of the epoxy-glass material. The conductor pattern is comprised of a multitude of fine line paths which terminate on relatively large surface land areas.
In order to provide electrical connections between vertically aligned land areas on the stacked layers, vias are drilled through the multilayer board at the central portions of the land areas and an electrically conductive material such as electroless plated copper is deposited in the via to make the desired interlayer connection. Vias 40 mils or greater in diameter can be accomplished easily with existing mechanical drills. However, with the advent of large-scale integration of circuits, greater density of inteconnection requires smaller diameter land areas (e.g., 25 mils) and smaller diameter vias (e.g., 10 to 15 mils). However, mechanical drills 10 to 15 mils in diameter are extremely fragile and subject to breakage which requires time-consuming and expensive replacement or repair.
One possible solution to the problem of drilling small diameter vias is the implementation of laser drilling. It is well-known to drill small diameter holes in epoxy-glass substrates using a CO.sub.2 laser; however, the wavelength (10.6 .mu.m) of that laser is reflected by copper, therefore, holes cannot be readily drilled therethrough. A YAG laser could be used to drill apertures in the copper, but that laser is not effective for drilling through the epoxy-glass insulation. Thus, by alternately using a YAG laser to drill through the copper layers and the CO.sub.2 laser to drill thrugh the epoxy-glass layers, a narrow diameter via could be drilled through the multilayer circuit board.
Obviously, such alternating use of different lasers would be most time consuming and uneconomical in a production environment. Accordingly, a need exists for a simple and effective technique for drilling small diameter vias through a circuit board.